The problem is, I don't really think you can replace motherboard in this dell with a non-dell one.
Dell uses custom built mainboards that not necessarily follow the ATX standard. This means that everything from screw holes to case backplate for IO ports can be incompatible with any board you can buy. Hell, even placement of components like CPU or RAM, which is different on a new board and may interfere with other components like HDD, ODD and other weird stuff dell installs in their cases.
Secondly, replacing your motherboard with something newer will mean you have to buy a DDR3 memory too. This means a couple bucks more.
I chose a good CPU, with cheap, but nice microATX board, 4GB RAM and one of the cheapest cases I could find. Feel free to change it to any that is more to your liking.
PCPartPicker part list /
Price breakdown by merchant /
Benchmarks
CPU: Intel Core i5-3350P 3.1GHz Quad-Core Processor ($177.98 @ SuperBiiz)
Motherboard: ASRock B75 PRO3-M Micro ATX LGA1155 Motherboard ($69.99 @ Amazon)
Memory: Corsair XMS3 4GB (2 x 2GB) DDR3-1600 Memory ($36.99 @ NCIX US)
Case: Topower TP-6208BB ATX Mid Tower Case ($24.99 @ Newegg)
Total: $309.95
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-08-15 03:59 EDT-0400)
However, if you do some thorough research and decide that this MB will fit in your current case, you can get 8GB of RAM with this budget.
For the same money the alternative is to look for used i5-2500k, should cost about $180, together with a used Z68 board (gen3) for about $60. This would be a little faster than 3350p+b75 and you could OC it even more. However, you'd need a better cooler for that.